1. Field of the Invention
This invention relates to an improved process for the fractionation of particles. More particularly, the invention relates to a new field-flow fractionation (FFF) process which uses a modified channel structure to improved the speed and effectiveness of operation.
Specifically, the invention provides a modified continuous flow FFF process for the separation of samples of particles and macromolecules which uses a modified channel structure to reduce the relaxation effect, reduce sample adhesion to the wall, and where possible eliminate the stop-flow procedure and thus greatly increase the speed and stability of operation. The new process of the invention comprises an improvement in the FFF process wherein a carrier fluid containing the particles to be separated is forced through a thin flow channel having an inlet and outlet end and a field or gradient is used to induce a driving force acting across the thin dimension perpendicular to the flow axis and the particles entering the channel at the inlet end undergo a relaxation process and approach a steady state distribution within the channel, the improvement comprising using as the thin channel a thin channel whose thickness is reduced at the inlet end for a substantial distance beyond the inlet means, such as the conventional triangular or near triangular piece, and then broadened out at the outlet end.
The invention further provides an apparatus for conducing the above-described process.
2. Prior Art
There is a growing need in industry and health sciences for the separation and characterization of micron sized particles including biological cells, latices, environmental particles, industrial powders, crystallization products, and related particulate matter. There is also a growing need for the separation of submicron sized particles, macromolecules and synthetic polymers.
Various methods have been proposed, but in general, they have been too slow, complex in operation, inefficient and expensive or have failed to effect the separation with the desired degree of resolution needed for commercial operations.
Some of the highest resolutions techniques disclosed have been those based on field-flow fractionation (FFF) as disclosed in the following U.S. patents and copending patent applications: U.S. Pat. No. 3,449,938, U.S. Pat. No. 4,147,621, U.S. Pat. No. 4,214,981, U.S. Pat. No. 4,250,026, U.S. Pat. No. 4,737,268, and copending patent applications--Giddings--"Lif-Induced Hyperlayer Field-Flow Fractionation Process for Particle Separation" Ser. No. 153,774, filed Feb. 8, 1988, U.S. patent application--Giddings--"Process for Continuous Particle and Polymer Separation in Split-Flow thin Cells using Flow-Dependent Lift Forces", Ser. No. 194,851, filed May 17, 1988, U.S. patent application--Giddings--"High Speed Separation of Ultra-High Molecular Weight Polymers by Hyperlayer Field-Flow Fractionation" Ser. No. 217,707, filed Jul. 11, 1988, and U.S. patent application--Williams--"Process of Programming of Field-Flow Fractionation"--Ser. No. 237,188, filed Aug. 29, 1988.
Attempts have been made to improve the FFF process, such as disclosed in Giddings et al--Anal. Chem. 56 2099) 1984, which discloses a method for reducing disturbances at the triangular end piece by reducing volume and thickness at the end piece.
The field-flow fractionation technique, however, has been limited for certain operations because of the problem as to speed of operation and the loss of sample material by adhesion to the wall for the following reasons.
When a particle sample first enters a field-flow fractionation channel, it is generally distributed broadly over the channel cross-section. Before normal sample migration can occur, the components of the sample must undergo a relaxation process in which they approach a steady-state distribution within the channel, usually by accumulating near one channel wall. This process requires a finite time (typically from 10 seconds to 30 minutes) described by the relaxation time .nu.. Because a good deal of band broadening can occur during the relaxation process, a stop-flow procedure is commonly used in which the flow through the channel is halted for a period of time .nu. in order to allow relaxation to occur under static conditions. The stop-flow procedure, generally required to avoid losses in resolution, is particularly essential for high flow rate runs. However, under any circumstances, stop-flow is an inconvenience and it consumes additional time for separation and often introduces baseline instabilities in sample detection. Sample losses due to adsorption or adhesion at the accumulation wall are also greatest during the stopflow period. Attempts to reduce the stopflow time by increasing the field strength only magnify the sample loss problem. Clearly, the development of a method to reduce relaxation effects and, where possible, eliminate the stop-flow procedure would represent an important advance in FFF techniques, particularly for high speed operations.
It is an object of the invention, therefore, to provide an improved FFF process which solves the above-noted problem as to relaxation time. It is a further object to provide a new FFF process which effects a reduction in the relaxation effect and where possible eliminates the stop-flow procedure. It is a further object to provide a modified FFF process which is capable of effecting separation at a very high rate of speed. It is a further object to provide a modified FFF process which can be adapted to any of the above-noted FFF techniques. These and other objects of the invention will be apparent from the following detailed description thereof.